Selective Bacterial Targeting and Infection-Triggered Release of Antibiotic Colistin Conjugates.

The treatment of life-threatening diseases with small molecule drugs often leads to a fragile balance between desired efficacy and undesired toxic effects. A prominent example concerns the natural lipopeptide colistin that has been re-introduced clinically as a 'last resort' antibiotic for the treatment of drug-resistant infections, in spite of strong and frequent nephrotoxic effects. In order to render such potent, but toxic antibiotics more selective, we have explored a novel conjugation strategy that includes drug accumulation followed by infection-triggered release of the drug. Bacterial targeting was achieved using a modified fragment of the human antimicrobial peptide ubiquicidin, as demonstrated by fluorophore-tagged variants. To limit the release of the effector colistin only to infection-related situations, we introduced a linker that was cleaved by neutrophil elastase (NE), an enzyme secreted by neutrophils at infection sites. The linker carried an optimized sequence of amino acids that was required to assure sufficient cleavage efficiency. The antibacterial activity of five regioisomeric conjugates prepared by total synthesis was masked, but was released upon exposure to recombinant NE when the linker was attached to amino acids at the 1- or the 3-position of colistin. A proof-of-concept was achieved in co-cultures of primary human neutrophils and Escherichia coli that induced the secretion of NE, the release of free colistin, and an antibacterial efficacy that was equal to that of free colistin.